1. Field of the Invention
The invention relates to a linear actuator comprising a reversible DC-motor, which through a transmission drives a spindle and thus conveys a linear movement to a spindle nut secured against rotation, where the spindle nut is connected to an activation element which is displaceable between two end positions on the spindle, a power supply for the DC-motor, an electrical control, at least one end stop switch for interrupting the current for the DC-motor in the end position of the activation element or signaling of the end stop position, and a position determination system for determining the position of the spindle nut on the spindle.
2. The Prior Art
For explanation, the invention derives from a linear actuator of the type comprising an activation element, which is axially displaceable in the longitudinal direction of the actuator. A linear actuator comprises an electric motor, which through a transmission drives a spindle. On the spindle there is a spindle nut, to which a tubular activation element is secured with one end. The other end of the activation element is secured by means of a front fixture to the movable element in the structure into which the actuator is incorporated while the actuator with a rear fixture is secured in the stationary part of the structure or vice versa. See WO 02/2984 A1 to Linak A/S. In WO 2005/122841 A1 the spindle nut and the activation element are alternatively designed as one piece which thus appears as a displaceable block on the spindle. When retaining the spindle nut, it will screw itself outwards or inwards on the spindle depending on the direction of rotation, and the activation element will thus be displaced axially outwards or be retracted (WO 96/12123 Koch/Okin).
An absolute determination of the position of the activation element may be carried out by means of a potentiometer, e.g., a rotary potentiometer, which through a gearing is driven by a motor. The location of the glider of the rotary potentiometer will change proportionally to the position of the activation element, and will thus, when the potentiometer is connected to an electrical control, be able to express the position of the activation element (EP 0 831 250 A2 Dana Corp.). A potentiometer in the shape of a sliding potentiometer positioned parallel to the spindle is also known. Here, the spindle nut is typically connected to the glider of the potentiometer (DE 10 2005 035 104 B4 Dewert Antriebs- and Systemtechnik).
Another and more common way to determine the position of the activation element, which due to financial and reliability reasons is preferred in actuators, is by counting the number of rotations of the spindle. The number of rotations multiplied by the thread pitch of the spindle determines the position of the activation element. When the gear ratio is known, the number of rotations of the spindle can be calculated. A method for determining the position of the activation element by counting the rotations is to use incremental sensors like, e.g., a magnetic encoder. A magnetic encoder comprises a magnet ring with a number of poles, which activates a Reed-switch or a Hall-sensor every time one of the poles passes the switch, alternatively the Hall-sensor, at which an electric signal is given (WO 2007/006313 A1 Linak A/S).
Alternatively to a magnetic encoder, an optical encoder may be usedfidi which functions in a similar way, i.e., with a rotating disc with perforations and a light source. When a perforation passes the light source, a beam of light is temporarily let through to a photo cell, which thus gives a signal. Based on the signals the position is calculated by means of a micro processor in the control.
The initiating of an incremental position determination system must be based on a known position of the spindle nut on the spindle, which may be one of the two end positions of the spindle nut's travel on the spindle, indicated by the two end stop switches.
The mentioned systems provide a fine position determination of the spindle nut, but still have limitations and disadvantages.
If a potentiometer is used, this is difficult to incorporate into the actuator as it, depending on whether it is a rotary or sliding potentiometer, must be equipped with an appropriate gearing or rod device, for transferring the movement of the spindle nut to the potentiometer. Further, a potentiometer is mechanically worn, so that it in time becomes unreliable or directly faulty. A potentiometer is further not a cheap component, and even less so when the mechanical connection to the spindle nut is considered.
In order to be able to provide a reliable position determination, an incremental position determination system, e.g., of the magnetic type, requires that the counting device is calibrated to the distance both in respect to the travel compared to rotations, but is also initiated with known positions of the spindle nut on the spindle. If the supply for such a system is interrupted, a new initiation must be performed in order to be able to rely completely on the position determination, as it cannot be known with certainty whether a manual adjustment of the actuator meanwhile has been performed, e.g., by release of a quick-release (WO 2007/006313 Linak A/S). Further, a pulse generator in the shape of a ring magnet must be moved proportionally to the rotation of the spindle, the rotation of the ring magnet is registered by a sensor. This requires that a tap is constructed for the pulse generator, which in order to obtain a more accurate position determination is often equipped with a gearing. An incremental position determination system is thus expensive as it both comprises many mechanical parts, i.e., a tap with a gearing, a pulse generator, at least one sensor, a counting circuit and a calculation device, typically a microprocessor for managing the task.
The purpose of the invention is to provide a solution to the outlined problem, which is both simpler and more inexpensive and further more reliable than the prior art—in particular a solution with a simpler mechanical structure, without expensive components and which is also more reliable over time. Further, a solution is desired which is not as vulnerable to power cuts.